Tropical nature, and other essays — A Reader’s Guide

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AUTHOR OF “THE MALAY ARCHIPELAGO,” “THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS,”“CONTRIBUTIONS TO THE THEORY OF NATURAL SELECTION,” ETC., ETC.

[_The Right of Translation and Reproduction is Reserved._]

R. CLAY, SONS, AND TAYLOR,

Land of the Sun! where joyous green-robed Spring And leaf-crowned Summer deck the Earth for ever; No Winter stern their sweet embrace to sever And numb to silence every living thing, But bird and insect ever on the wing, Flitting ’mid forest glades and tangled bowers, While the life-giving orb’s effulgent beams Through all the circling year call forth the flowers. Here graceful palms, here luscious fruits have birth; The fragrant coffee, life-sustaining rice, Sweet canes, and wondrous gums, and odorous spice; While Flora’s choicest treasures crowd the teeming earth. Beside each cot the golden Orange stands, And broad-leaved Plantain, pride of Tropic lands.

Sweet changing Seasons! Winter cold and stern, Fair Spring with budding leaf and opening flower, And Summer when the sun’s creative power Brings leafy groves and glades of feathery fern, The glorious blossoms of sweet-scented May, The flowery hedgerows and the fragrant hay, And the wide landscape’s many-tinted sheen. Then Autumn’s yellow woods and days serene; And when we’ve gathered in the harvest’s treasure, The long nights bring us round the blazing hearth, The chosen haunt of every social pleasure. Land of green fields and flowers! Thou givest birth To shifting scenes of beauty, which outshine Th’ unvarying splendours of the Tropic’s clime.

The luxuriance and beauty of Tropical Nature is a well-worn theme, and there is little new to say about it. The traveller and the naturalist have combined to praise, and not unfrequently to exaggerate the charms of tropical life--its heat and light, its superb vegetable forms, its brilliant tints of flower and bird and insect. Each strange and beautiful object has been described in detail; and both the scenery and the natural phenomena of the tropics have been depicted by master hands and with glowing colours. But, so far as I am aware, no one has yet attempted to give a general view of the phenomena which are _essentially_ tropical, or to determine the causes and conditions of those phenomena. The local has not been separated from the general, the accidental from the essential; and, as a natural result, many erroneous ideas have become current as to what are really the characteristics of the tropical as distinguished from the temperate zones.

In the present volume I have attempted to supply this want; and for my materials have drawn chiefly on my own twelve years’ experience of the eastern and western tropics of the equatorial zone, where the characteristic phenomena of tropical life are fully manifested.

So many of the most remarkable forms of life are now restricted to the tropics, and the relations of these to extinct types which once inhabited the temperate zones open up so many interesting questions as to the past history of the earth, that the present inquiry may be considered a necessary preliminary to a study of the problem--how to determine the climates of geologic periods from the character of their organic remains. This part of the subject is however both complex and difficult, and I have only attempted to indicate what seem to me the special physical conditions to which the existing peculiarities of tropical life are mainly due.

The three opening chapters treat the subject under the headings of climate, vegetation, and animal life. The conditions and causes of the equatorial climate are discussed in some detail, and the somewhat complex principles on which it depends are popularly explained. In the chapters on plant and animal life, the general aspects and relations of their several component elements have been dwelt upon; all botanical and zoological details and nomenclature being excluded, except so far as was absolutely necessary to give precision to the descriptions and to enable us to deduce from them some conclusions of importance.

The remaining chapters have all a more or less direct connection with the leading subject. The family of humming-birds is taken as an illustration of the luxuriant development of allied forms in the tropics, and as showing the special mode in which natural selection has acted to bring about considerable changes in a limited period. The discussion on the nature and origin of the colours of animals and plants, is intended to show how far and in what way these are dependent on the climate and physical conditions of the tropics. The chapter entitled “By-paths in the Domain of Biology” contains an account of certain curious relations of colour to locality, which are almost exclusively manifested within the tropical zones; while the essay on “Distribution of Animals and Geographical Changes,” elucidates the relations of the several continents in past time, and the probable origin of many of the groups now characteristic of tropical or of temperate regions.

While discussing the general laws and phenomena of colour in the organic world, and its special developments among certain groups of animals, I have been led to a theory of the diverse colours of the sexes and of the special ornaments and brilliant hues which distinguish certain male birds and insects, which is directly opposed to the view held by Mr. Darwin and so well explained and illustrated in his great work on “The Descent of Man and on Selection in Relation to Sex.” Being strongly impressed with the importance and fundamental truth of this theory, I published my first sketch of the subject in _Macmillan’s Magazine_ in order that it might have the benefit of criticism before making it public in a more permanent form. Taking advantage of some suggestions from Mr. Darwin and from a few other correspondents, I have made considerable additions to the original essay and have rearranged, and I trust strengthened the argument, which I now hope may attract the attention of all who are interested in the subject. I may be allowed here to remark, that my theory cannot be properly understood without reading the whole chapter on “The Colours of Animals;” because the view set forth and illustrated in the first part of that chapter--that colour in nature is normal, and that its presence hardly requires to be accounted for so much as its absence--is an essential part of the theory.

CROYDON, _April, 1878_.

I. The Climate and Physical Aspects of the Equatorial Zone.

The three Climatal Zones of the Earth--Temperature of the Equatorial Zone--Causes of the Uniform High Temperature near the Equator--Influence of the Heat of the Soil--Influence of the Aqueous Vapour of the Atmosphere--Influence of Winds on the Temperature of the Equator--Heat due to the Condensation of Atmospheric Vapour--General Features of the Equatorial Climate--Uniformity of the Equatorial Climate in all Parts of the Globe--Effects of Vegetation on Climate--Short Twilight of the Equatorial Zone--The Aspect of the Equatorial Heavens--Intensity of Meteorological Phenomena at the Equator--Concluding Remarks _pages 1-26_

II. Equatorial Vegetation.

The Equatorial Forest-belt and its Causes--General Features of the Equatorial Forests--Low-growth Forest-trees--Flowery Trunks and their Probable Cause--Uses of Equatorial Forest-trees--The Climbing Plants of the Equatorial Forests--Palms--Uses of Palm-trees and their Products--Ferns--Ginger-worts and Wild Bananas--Arums--Screw-Pines--Orchids--Bamboos--Uses of the Bamboo--Mangroves--Sensitive-plants--Comparative Scarcity of Flowers--Concluding Remarks on Tropical Vegetation _pages 27-68_

III. Animal Life in the Tropical Forests.

Difficulties of the Subject--General Aspect of the Animal Life of Equatorial Forests--Diurnal Lepidoptera or Butterflies--Peculiar Habits of Tropical Butterflies--Ants, Wasps, and Bees--Ants--Special Relations between Ants and Vegetation--Wasps and Bees--Orthoptera and other Insects--Beetles--Wingless Insects--General Observations on Tropical Insects--Birds--Parrots--Pigeons--Picariæ--Cuckoos--Trogons, Barbets, Toucans and Hornbills--Passeres--Reptiles and Amphibia--Lizards--Snakes--Frogs and Toads--Mammalia--Monkeys--Bats--Summary of the Aspects of Animal Life in the Tropics _pages 69-123_

IV. Humming-Birds: as Illustrating the Luxuriance of Tropical Nature.

Structure--Colours and Ornaments--Descriptive Names--The Motions and Habits of Humming-Birds--Display of Ornaments by the Male--Food--Geographical Distribution and Variation--Humming-Birds of Juan Fernandez as illustrating Variation and Natural Selection--The Relations and Affinities of Humming-Birds--How to Determine Doubtful Affinities--Resemblances of Swifts and Humming-Birds--Differences between Sun-Birds and Humming-Birds _pages 124-157_

V. The Colours of Animals and Sexual Selection.

General Phenomena of Colour--Theory of Heat and Light as producing Colour--Changes of Colour in Animals produced by Coloured Light--Classification of Organic Colours--Protective Colours--Warning Colours--Sexual Colours--Typical Colours--The Nature of Colour--How Animal Colours are Produced--Colour a Normal Product of Organization--Theory of Protective Colours--Theory of Warning Colours--Imitative Warning Colours--The Theory of Mimicry--Theory of Sexual Colours--Colour as a Means of Recognition--Colour proportionate to Integumentary Development--Selection by Females not a Cause of Colour--Probable Use of the Horns of Beetles--Cause of the greater Brilliancy of some Female Insects--Origin of the Ornamental Plumage of Male Birds--Theory of the Display of Ornaments by Males--Natural Selection as neutralizing Sexual Selection--Greater Brilliancy of some Female Birds--Colour-development as illustrated by Humming-Birds--Theory of Typical Colours--Local Causes of Colour-development--Summary on Colour-development in Animals--Concluding Remarks on Causes of Bright Colour in the Tropics _pages 158-220_

VI. The Colours of Plants and the Origin of the Colour-Sense.

Source of Colouring-matter in Plants--Protective Coloration and Mimicry in Plants--Attractive Colours of Fruits--Protective Colours of Fruits--Seeds how Protected--Attractive Colours of Flowers--Attractive Odours in Flowers--Attractive Grouping in Flowers--Why Alpine Flowers are so Beautiful--Why Allied Species of Flowers differ in Size and Beauty--Absence of Colours in Wind-fertilized Flowers--The same Theory of Colour applicable to Animals and Plants--Relation of the Colours of Flowers and their Geographical Distribution--Recent Views as to the Direct Action of Light on the Colours of Flowers and Fruits--Concluding Remarks on the Importance of Colour in the Organic World--The Origin of the Colour-sense.--Supposed Increase of Colour-perception within the Historical Period--Concluding Remarks on the Colour-sense _pages 221-248_

VII. BY-PATHS IN THE DOMAIN OF BIOLOGY.

BEING AN ADDRESS DELIVERED TO THE BIOLOGICAL SECTION OF THE BRITISH ASSOCIATION (GLASGOW, SEPTEMBER 6TH, 1876) AS PRESIDENT OF THE SECTION.

Introductory Remarks--ON SOME RELATIONS OF LIVING THINGS TO THEIR ENVIRONMENT.--The Influence of Locality on Colour in Butterflies and Birds--Sense-perception influenced by Colour of the Integuments--Relations of Insular Plants and Insects--RISE AND PROGRESS OF MODERN VIEWS AS TO THE ANTIQUITY AND ORIGIN OF MAN--Indications of Man’s Extreme Antiquity--Antiquity of Intellectual Man--Sculptures on Easter-Island--North American Earthworks--The Great Pyramid--Conclusion _pages 249-303_

VIII. The Distribution of Animals as Indicating Geographical Changes.

Old Opinions on Continental Changes--Theory of Oceanic Islands--Present and Past Distribution of Land and Sea--Zoological Regions--The Palæarctic Region--The Ethiopian Region--The Oriental Region--Past changes of the Great Eastern Continent--Regions of the New World--Past History of the American Continents--The Australian Region--Summary and Conclusion _pages 304-347_

THE CLIMATE AND PHYSICAL ASPECTS OF THE EQUATORIAL ZONE.

The three Climatal Zones of the Earth--Temperature of the Equatorial Zone--Causes of the Uniform High Temperature near the Equator--Influence of the Heat of the Soil--Influence of the Aqueous Vapour of the Atmosphere--Influence of Winds on the Temperature of the Equator--Heat due to the Condensation of Atmospheric Vapour--General features of the Equatorial Climate--Uniformity of the Equatorial Climate in all parts of the globe--Effects of Vegetation on Climate--Short Twilight of the Equatorial Zone--The aspect of the Equatorial Heavens--Intensity of meteorological phenomena at the Equator--Concluding Remarks.

It is difficult for an inhabitant of our temperate land to realize either the sudden and violent contrasts of the arctic seasons or the wonderful uniformity of the equatorial climate. The lengthening or the shortening days, the ever-changing tints of spring, summer, and autumn, succeeded by the leafless boughs of winter, are constantly recurring phenomena which represent to us the established course of nature. At the equator none of these changes occur; there is a perpetual equinox and a perpetual summer, and were it not for variations in the quantity of rain, in the direction and strength of the winds, and in the amount of sunshine, accompanied by corresponding slight changes in the development of vegetable and animal life, the monotony of nature would be extreme.

In the present chapter it is proposed to describe the chief peculiarities which distinguish the equatorial from the temperate climate, and to explain the causes of the difference between them,--causes which are by no means of so simple a nature as are usually imagined.

The three great divisions of the earth--the tropical, the temperate, and the frigid zones, may be briefly defined as the regions of uniform, of variable, and of extreme physical conditions respectively. They are primarily determined by the circumstance of the earth’s axis not being perpendicular to the plane in which it moves round the sun; whence it follows that during one half of its revolution the north pole, and during the other half the south pole, is turned at a considerable angle towards the source of light and heat. This inclination of the axis on which the earth rotates is usually defined by the inclination of the equator to the plane of the orbit, termed the obliquity of the ecliptic. The amount of this obliquity is 23½ degrees, and this measures the extent on each side of the equator of what are called the tropics, because within these limits the sun becomes vertical at noon twice a year, and at the extreme limit once a year, while beyond this distance it is never vertical. It will be evident, however, from the nature of the case, that the two lines which mark the limits of the geographical “tropics” will not define any abrupt change of climate or physical conditions, such as characterise the tropical and temperate zones in their full development. There will be a gradual transition from one to the other, and in order to study them separately and contrast their special features we must only take into account the portion of each in which these are most fully exhibited. For the temperate zone we may take all countries situated between 35° and 60° of latitude, which in Europe will include every place between Christiania and Algiers, the districts further south forming a transitional belt in which temperate and tropical features are combined. In order to study the special features of tropical nature, on the other hand, it will be advisable to confine our attention mainly to that portion of the globe which extends for about twelve degrees on each side of the equator, in which all the chief tropical phenomena dependent on astronomical causes are most fully manifested, and which we may distinguish as the “equatorial zone.” In the debateable ground between these two well contrasted belts local causes have a preponderating influence; and it would not be difficult to point out localities within the temperate zone of our maps, which exhibit all the chief characteristics of tropical nature to a greater degree than other localities which are, as regards geographical position, tropical.

_Temperature of the Equatorial Zone._--The most characteristic, as it is the most important feature in the physical conditions of the great equatorial zone is the wonderful uniformity of its temperature, alike throughout the changes of day and night, and from one part of the year to another. As a general rule, the greatest heat of the day does not exceed 90° or 91° Fahr., while it seldom falls during the night below 74° Fahr. It has been found by hourly observations carried on for three years at the meteorological observatory established by the Dutch government at Batavia, that the extreme range of temperature in that period was only 27° Fahr., the maximum being 95° and the minimum 68°. But this is, of course, very much beyond the usual daily range of the thermometer, which is, on the average, only a little more than 11° Fahr.; being 12·6° in September when it is greatest, and only 8·1° in January, when it is least.

Batavia, being situated between six and seven degrees south of the equator, may be taken as affording a fair example of the climate of the equatorial zone; though, being in an island, it is somewhat less extreme than many continental localities. Observations made at Para, which is continental and close to the equator, agree however very closely with those at Batavia; but at the latter place all the observations were made with extreme care and with the best instruments, and are therefore preferred as being thoroughly trustworthy.[1] The accompanying diagram, showing by curves the monthly means of the highest and lowest daily temperatures at Batavia and London, is very instructive; more especially when we consider that the maximum of temperature is by no means remarkably different in the two places, 90° Fahr, being sometimes reached with us and not being often very much exceeded at Batavia.

[1] “Observations Made at the Magnetical and Meteorological Observatory at Batavia. Published by order of the Government of Netherlands India. Vol. I. Meteorological, from Jan. 1866 to Dec. 1868; and Magnetical, from July 1867 to June 1870. By Dr. P. A. Bergsma. Batavia, 1871.” This fine work is entirely in English.

_Causes of the Uniform High Temperature near the Equator._--It is popularly supposed that the uniform high temperature of the tropics is sufficiently explained by the greater altitude, and therefore greater heating-power, of the midday sun; but a little consideration will show that this alone by no means accounts for the phenomenon. The island of Java is situated in from six and a half to eight and a half degrees of south latitude, and in the month of June the sun’s altitude at noon will not be more than from 58° to 60°. In the same month at London, which is fifty-two and a half degrees of north latitude, the sun’s noonday altitude is 62°. But besides this difference of altitude in favour of London there is a still more important difference; for in Java the day is only about eleven and a half hours long in the month of June, while at London it is sixteen hours long, so that the total amount of sun-heat received by the earth must be then very much greater at London than at Batavia. Yet at the former place the mean temperature of the day and night is under 60° Fahr., while in the latter place it is 80° Fahr., the daily maximum being on the average in the one case about 68° and in the other about 89°.

Neither does the temperature at the same place depend upon the height of the sun at noon; for at Batavia it is nearly vertical during October and February, but these are far from being the hottest months, which are May, June, and September; while December, January, and February are the coldest months, although then the sun attains nearly its greatest altitude. It is evident, therefore, that a difference of 30° in the altitude of the sun at noon has no apparent influence in raising the temperature of a place near the equator, and we must therefore conclude that other agencies are at work which often completely neutralise the effect which increased altitude must undoubtedly exert.

There is another important difference between the temperate and tropical zones, in the direct heating effect of the sun’s rays independently of altitude. In England the noonday sun in the month of June rarely inconveniences us or produces any burning of the skin; while in the tropics, at almost any hour of the day, and when the sun has an elevation of only 40° or 50°, exposure to it for a few minutes will scorch a European so that the skin turns red, becomes painful, and often blisters or peels off. Almost every visitor to the tropics suffers from incautious exposure of the neck, the leg, or some other part of the body to the sun’s rays, which there possess a power as new, as it is at first sight inexplicable, for it is not accompanied by any extraordinary increase in the temperature of the air.

These very different effects, produced by the same amount of sun-heat poured upon the earth in different latitudes is due to a combination of causes. The most important of these are, probably,--the constant high temperature of the soil and of the surface-waters of the ocean,--the great amount of aqueous vapour in the atmosphere,--the great extent of the intertropical regions which cause the winds that reach the equatorial zone to be always warm,--and the latent heat given out during the formation of rain and dew. We will briefly consider the manner in which each of these causes contributes to the degree and the uniformity of the equatorial temperature.